Great variability in sex-typed behaviors occurs within a given sex as well as between the sexes. Both genetic and social factors have been theorized to account for these intra- and inter-sex differences in behavior. Although these differences can be quantified and are evident from an early age, their determinants are still incompletely understood. This study focuses on same-sex twin children who are reported by their mothers to be either similar or divergent in conventional sex-role behaviors. Our sample of discordant and concordant monozygotic twin children have been identified and selected through a national behavioral preference survey questionnaire of over 700 pairs of twins between the ages of 4 and 11. The restriction of using only monozygotic twins in this study greatly increases our capacity to control for genetic variability so that behavioral differences and similarities between co-twins can more clearly be attributed to social factors. Our sample consists of twelve pairs of twin children behaviorally discordant on sex-typed variable (6 male pairs and 6 female pairs) who have been demographically matched with twelve pairs of twins who are behaviorally concordant for sex-typed behaviors. Since controllig for the influence of genetic variability is critical for this study, all 24 pairs of reportedly monozygotic twins will have their zygosity confirmed through a laboratory blood-grouping analysis before data collection is begun. Data on confirmed monozygotic co-twins will be collected from four sources, 1) parent questionnaires, 2) structured clinical interviews with parents and co-twins, 3) structured behavioral play observations, and 4) psychological testing of co-twins. These case study methods of data collection will be used to assess various social factors which may account for differences and similarities in behavior between co-twins. Analysis of data obtained from these methods will provide an assessment of social influences behind similar and dissimilar psychosexual behaviors of genetically similar male and female children and increase our understanding of the general processes of psychosexual differentiation.